Given two integers n
and k
, find how many different arrays consist of numbers from 1
to n
such that there are exactly k
inverse pairs.
We define an inverse pair as following: For ith
and jth
element in the array, if i
< j
and a[i]
> a[j]
then it‘s an inverse pair; Otherwise, it‘s not.
Since the answer may be very large, the answer should be modulo 109 + 7.
Example 1:
Input: n = 3, k = 0 Output: 1 Explanation: Only the array [1,2,3] which consists of numbers from 1 to 3 has exactly 0 inverse pair.
Example 2:
Input: n = 3, k = 1 Output: 2 Explanation: The array [1,3,2] and [2,1,3] have exactly 1 inverse pair.
时间: 2024-10-15 14:59:13